What is a vaginal yeast infection caused by?

The vagina contains bacteria that regulate the amount of yeast. If the amount of these bacteria is affected and an imbalance occurs, an excess of these yeast cells will grow.
Common causes of bacterial imbalance include taking antibiotics, a change in estrogen levels due to pregnancy or hormone replacement therapy, diseases such as diabetes and HIV, or if you are taking birth control, menstruating, or allow sperm to enter the vagina during sex. Women who are pregnant or have experienced menopause are also more likely to get yeast infections.